Method and apparatus for monitoring scanning conditions during control of a yarn feeding device

ABSTRACT

A method and apparatus for monitoring the scanning conditions when controlling a yarn feeding device including a storing surface for the yarn, a drive motor, a sensor device having at least one sensor oriented towards a scanning zone defined in the yarn feeding device, and a control circuit connected to the sensor device. The sensor generates an object-output signal for control purposes in response to the movement, absence or presence of an object in the scanning zone. A test signal is formed from and substantially synchronously with the object-output signal, and the signal level of the test signal is compared with an alarm threshold value representing a just barely acceptable deterioration level of the scanning conditions. An alarm signal is generated when the signal level of the test signal falls below a predetermined threshold value.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for monitoringscanning conditions during the control of a yarn feeding deviceincluding a sensor device oriented towards a scanning zone defined on ayarn storing surface. The sensor device senses motion or the presence orabsence of an object in the scanning zone, and a control circuitprocesses an output signal of the sensor device to control a drive motorfor replenishing the yarn storing surface with yarn.

BACKGROUND OF THE INVENTION

Such a method is disclosed in U.S. Pat. No. 4,865,085 (corresponding toEP-0 199 059 BI). In this method, the sensor device operates with areceiver which monitors the axial movement of yarn windings on astationary storing drum, and a second receiver monitors the quality ofthe light transmission. An output signal of the second receiver iscompared with a threshold value in order to provide an additional usefulsignal which serves to increase the light intensity for both receiverswhen the light transmission has deteriorated. Also, an alarm signal foran operator can be generated indicating the necessity for cleaning ofthe light transmission path by removing contaminants which disturb orblock the light transmission.

In a method disclosed in U.S. Pat. No. 4,963,757, a light source feedstwo receivers, one of which scans a yarn and the other scans only thelight transmission quality in order to maintain the relation between theoutput signals of both receivers substantially constant, and tocompensate for a deterioration of the light transmission quality.

According to a method disclosed in U.S. Pat. No. 3,907,440, phase-offsetlight pulses for one receiver are generated by means of two pulsed lightsources, and a yarn is scanned only with the light pulses of one of thelight sources. The output signals originating from the light pulses notused for yarn scanning are compared with a nominal signal value in orderto maintain a predetermined relation between both signals and tocompensate for disturbing influences.

GB-A-22 27 092 discloses an optoelectronic sensor which consists of alight source and a receiver. The sensor is checked in a bank-notereceiving and discharging device as to the instantaneous scanningcharacteristics before the sensor takes part in the checking of a banknote. In a test routine, a state, as may later be found in testing abank note, is simulated by darkening the light source at the controlside, as compared with the normal light intensity of the light source.The level of resulting output signals of the receiver is compared with athreshold value level calculated by the control device from those outputsignals of the receiver that are obtained without and with the darkeningaction. If the level of the darkened output signal is below thethreshold value level, an alarm will be initiated.

According to another method known from WO95/16628 and used forcontrolling the drive motor of a yarn feeding device for a knittingmachine, the yarn feeding device includes a rotatably driveable storingdrum defining a storing surface and a stationary sensor device.Circumferentially offset surface areas of the storing surface aresimultaneously optoelectronically scanned in the scanning zone by meansof a plurality of sensors. In case where yarn is present in the scanningzone, the sensors simultaneously output identical output signals. Incontrast, when yarn is absent from the scanning zone, the sensorssimultaneously generate different output signals. By discriminationbetween the output signals, control signals are derived, and the drivemotor is driven as long as the scanning zone is free from yarn, untilyarn reaches the scanning zone again. When replenishing the yarn store,i.e., in the driven state of the drive motor, a speed signal for thecontrol circuit is derived from the output signal of a sensor. Apredetermined quality of the light transmission is required for theoperation of the sensor device. Contaminants and lint, which unavoidablyoccur when processing yarns, deteriorate the quality of the lighttransmission with increasing duration of operation. The sensor devicethen fails and the storing surface becomes empty, and this might lead toa defect in the product produced in the textile machine which is beingsupplied with yarn by the yarn feeding device. Therefore, it iscustomary operator cleans the light transmission path within periodsbased upon experience, e.g. by pressurized air or by sweeping. However,the cleaning steps are then carried out more often than necessary, or adisturbance occurs due to a lack of care of the operator.

It is the object of the present invention to provide a method of thekind as disclosed above, as well as a yarn feeding device which enablereliable detection of deteriorated scanning conditions which just barelyallow correct operation of the sensor device in a structurally simpleway and with a simple circuitry technique. The invention also enableselimination of these less than optimal scanning conditions so as toavoid defects in the product produced by the textile machine suppliedwith yarn by the yarn feeding device.

In the method according to the invention, an object-output signalgenerated for control purposes is also used to check the quality of thescanning conditions, e.g. the quality of the light transmission, bymeans of surface areas of the storing surface and/or a yarn which serveas the object. This does not require any appreciable additionalcomponents in the sensor device, or at the storing surface. The scanningconditions (or the light transmission quality) decisive for the functionof the sensor device are examined in the scanning zone, i.e. at theexact location where they are decisive for the function of the sensordevice for controlling the drive motor, and not at a location which isdistant from the scanning zone. A deterioration of the scanningconditions will change the signal level of the output signal, and alsothe signal level of the test signal which is compared with a thresholdvalue. The threshold value is set so as to correspond to a just barelyacceptable deterioration of the scanning conditions. When the testsignal finally falls below the threshold value the alarm signal isactivated. By means of the alarm signal, an operator becomes alarmedjust in time, i.e. neither too early nor too late, to clean theoperating area of the sensor device, i.e. for example the lighttransmitting path. However, the alarm signal can also be used toautomatically activate a cleaning device for the sensor device, whichcleaning device automatically carries out a cleaning step, e.g. byblowing away or sweeping away contaminants.

In the yarn feeding device, an examination of the scanning conditions ismade exactly at the location at which the object is scanned, i.e. at alocation where the quality of the scanning condition is of decisiveimportance for correct functioning of the sensor device. Since theobject-output signal itself is used as a basis for the test signal, noadditional sensor components or auxiliary means are needed at thestoring surface. Components already used for scanning the object arealso used for the test routine, and as a result, the scanning conditionis checked during operation periods only, and if scanning conditionsdeteriorate during an operation period, an alarm signal is generatedwhich alerts the operating personnel to remove the disturbance isgenerated, during which operation periods a deterioration of thescanning condition might disturb the operation of the sensor device.Further, the examination is not carried out permanently, i.e. it is notcarried out during unimportant time periods in which the scanningcondition is of no influence on the operation of the sensor device. Thestructural features provided are advantageous with yarn feeding deviceshaving a storing surface driven by the drive motor (rotatably drivenstoring body) as well as with yarn feeding devices having a stationarystoring surface during operation (stationary storing drum and rotatablydriven winding element), in order to reliably determine when adisturbance has to be eliminated.

The method according to the invention also includes monitoring the testsignal and the object-output signal, and a simple logical evaluation ofthe occurrence or the non-occurrence of both signals is carried out inorder to generate the alarm signal at a correct point in time and on thebasis of a correctly determined scanning condition.

Further, the test signal, as well as a speed signal for control purposesof the drive motor, are generated from the object-output signal. Anexamination of the scanning conditions is only carried out in the eventthat the drive motor must be driven when there is danger of emptying thestoring surface. Although the alarm signal is generated when the testsignal fails to appear, the speed signal still appears for unobstructeduse.

According to another aspect of the invention, the output signal and thetest signal are both compared with separate threshold values. The higherthreshold value represents a just barely acceptable deterioration of thescanning conditions. The output signal and the test signal not onlyoccur synchronously with one another, but are also identical in theirsignal levels which is decisive for comparison with the respectivethreshold value. Since the threshold value for the test signal ishigher, the test signal fails to appear as soon as the just barelyacceptable deterioration has occurred. The output signal is stillpresent and can be used in the predetermined way for control purposes.In the absence of the test signal, however, the alarm signal isgenerated. The lower threshold value can suitably be set to correspondto a worse deterioration of the scanning conditions (as compared to thethreshold value for the test signal) at which a correct operation of thesensor device is no more assured. In the event that the operator has notreacted to the alarm signal, the yarn feeding device, and appropriatelyalso the textile machine supplied with yarn by the yarn feeding device,can be switched off when the output or speed signal also fails toappear, in order to avoid emptying of the storing surface.

Alternatively, both signals can be compared with the same thresholdvalue. Prior to this comparison, the signal level of the test signal ischanged so that upon the comparison of its signal level with thealarm-threshold value, precise information is gained indicating the needof an alarm signal.

The method is particularly useful with opto-electronical and contactlessscanning in a yarn feeding device comprising an opto-electronic sensordevice predictable relation between the signal level and the quality ofthe light transmission.

However, the application of this method, and the structural features forcarrying out the method are not limited to optoelectronical scanning,and it is possible to use an output signal generated for a predeterminedcontrol purpose also for a testing routine with other contactlessscanning modes (sound, induction, etc) and even with contact yarnscanning. It is, however, important that the output signal used for thetest routine originates from the scanning of the object in the scanningzone and shows an easily evaluatable signal level which changes with adeterioration of the scanning conditions, e.g. due to dirt or dustdeposits. The principle of the invention is also useful for yarn feedingdevices having a stationary storing surface for the yarn. In thissituation, the output signal need not necessarily be in the form of asignal chain, even though this might be advantageous in some cases.

With the yarn feeding device according to the invention, theobject-output signal is used for the testing routine. The object-outputsignal represents the rotational speed of the drum and occursexclusively when the drum is driven due to absence of yarn in thescanning zone. By means of the test signal which is formed from theobject-output signal, the alarm signal can be generated simply andreliably, and exactly at a point in time when the scanning conditionshave deteriorated accordingly. One particularly useful feature of theinvention is that the operation of the sensor device is only checkedwhen the drive motor is driven for replenishing the yarn store. In sucha case, there is the risk of emptying the storing surface, because theboundary of the yarn store trails behind the scanning zone due toconsumption. If the drive motor is not driven, no test routine iscarried out. This is then uncritical, because there is a sufficientlybig yarn store already on the storing surface, which yarn store extendsinto the scanning zone. Elimination of the disturbance or cleaning iscarried out expediently when the drive motor is at a standstill, so thatthe yarn feeding device does not have to be switched off, and theproduction process of the textile machine, which is supplied with yarnby the yarn feeding device, does not have to be interrupted.

According to one embodiment of the yarn feeding device, despite theabsence of the test signal, the still occurring output signal is used asa speed signal for control purposes, and the alarm signal is generatedseparately. It is useful to use the microprocessor of the control deviceof the yarn feeding device (which microprocessor is typically providedin any event) as a combining or monitoring means for the above purpose,because the microprocessor usually has sufficient capacity for thisadditional program routine and thus only requires a software adaptation.

Further, in accordance with another embodiment, the microprocessorswitches off the yarn feeding device, and, expediently, also the textilemachine supplied with yarn from the yarn feeding device, via a switch orswitch-off member, namely as soon as the speed signal compared with thethreshold value also fails to appear. The invention thus incorporates adual-safety feature in the event that the operator fails to respond tothe alarm signal and eliminate the disturbance.

In another embodiment, a voltage divider produces the same signal levelfor the output signal and the test signal, and two comparators comparethe two signal levels with two different threshold values. As aconsequence, the speed signal which may possibly be needed for controlpurposes still occurs, even when the scanning conditions havedeteriorated to a just barely acceptable degree, although the testsignal has failed to appear and the alarm signal is generated.

In contrast, in an alternative embodiment, the signal level for the testsignal is changed in relation to the signal level of the output signalalready in the voltage divider. The speed signal which may possibly beneeded for control purposes can still be derived from the output signal,while with a just barely acceptable deterioration of the scanningconditions, the test signal fails to appear and the alarm signal isgenerated.

In another embodiment, a very reliable, preferably opto-electronical,yarn scanning with a precise control of the drive motor is achieved byproviding the plurality of single sensors, with only the output signalof one of the single sensors being used for the test routine.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to thedrawings, in which:

FIG. 1 is a longitudinal cross-sectional view of a yarn feeding device;

FIG. 2 is a horizontal cross-sectional view taken generally along line2--2 in FIG. 1.

FIG. 3 is a schematic or block diagram of a control circuit;

FIG. 4 is a detail view of a variant of the FIG. 3 control circuit; and

FIGS. 5, 5A, 5B, and 5C are schematic U/t signal diagrams.

DETAILED DESCRIPTION

A yarn feeding device F according to FIG. 1, particularly a yarn feedingdevice for a knitting machine, includes a housing 13 for an electricdrive motor 15 for rotatably driving a drum 1 via a shaft 16. Within aholding bracket 13' secured to housing 13, an opto-electronic sensingdevice 7 is provided. The sensing device 7 contains (FIG. 2) a pluralityof sensors S which are arranged in a spaced-apart manner in thecircumferential direction and are oriented towards a scanning zone (12)(shown in dash-dotted fashion). The sensor device 7 is, for instance,adjustable in a direction parallel to the drum axis. Sensor device 7 isconnected via a control circuit L to a control device C of drive motor15. Each of the sensors S may consist of a light source of its own, e.g.infra-red light, and a receiver, e.g. a photodiode which responds toreflected light.

Drum 1 defines a storing surface 2 for a yarn store 5 consisting ofwindings 6 of yarn Y which is withdrawn overhead of drum 1 and consumedupon demand by the textile machine (not shown), e.g. a knitting machine.Yarn Y is supplied to drum 1 in an upper region of drum 1 in FIG. 1 andis wound up by the rotation of drum 1, the drive motor 15 beingcontrolled such that despite a varying consumption of the yarn 5, themotor 15 serves to maintain the yarn store 5 such that the yarn store 5extends to the scanning zone 12. If yarn is present in the scanning zone12, the drive motor 15 is stopped or decelerated. If there is no yarnpresent in the scanning zone 12, the drive motor 15 is driven oraccelerated. Via control device C, the drive speed of the drive motor 15is approximately adapted to correspond to the yarn consumption.

Drum 1 can be designed as a bar cage with longitudinally extending barsR separated by interspaces Z. Instead of clear interspaces Z,longitudinal grooves could also be provided in drum 1, said groovesopening outwardly. Furthermore, it is possible to use a drum 1 with asmooth surface having surface areas A, B which alternate with oneanother in the circumferential direction and which have clearlydifferent, e.g. optical, scanning properties. In the illustratedembodiment, bars R and interspaces Z define first and secondcircumferential sections 8, 9 with clearly different scanning propertiesfor the sensors 5 of sensor device 7. The surface areas A, B ought to bedistributed regularly in circumferential direction. The sensor device 7of the shown embodiment contains three sensors S which are spaced fromeach other in the circumferential direction such that at least onesensor S scans a first circumferential section 8 while at least a secondsensor S simultaneously scans a second circumferential section 9.

In drum 1 a spoke star 19 is provided as an advance element G, with thespokes 18 of the spoke star 19 extending through the interspaces Z to arotational bearing 17 on shaft 16. The rotational bearing 17 and thespoke star 19 are inclined in relation to the axis 3 of drum 1. Sincethe rotational bearing 17 is mounted on a collar 17a which is preventedfrom rotating with shaft 16, the spoke star 19 moves the yarn store 5axially forward in a direction towards the scanning zone 12.Alternatively, a similar advance effect could be achieved by a conicaldesign of the drum 1 at the yarn feed side.

The sensors S are jointly provided in a housing 30. Light-transparentcovering screens 31, or a covering window shared by all sensors S,protect the sensors S against direct contamination. Contaminants may bedeposited on or in front of the covering screens 31 or the coveringwindow, and/or in the scanning zone 12 of drum 1.

FIG. 3 schematically illustrates in a block diagram a possibleembodiment of a control circuit L for generating drive control signalsfor drive motor 15 on the basis of the output signal of sensor device 7,or on the basis of the output signals of the sensors S.

The sensors S consist of transmitters D7, D8 and D9 and receiverelements T1, T2 and T3, preferably operating with infra-red light. Thesensors, the receivers and operational amplifiers 20, 21 and 22cooperating therewith, are jointly connected to a source of constantvoltage. The infra-red radiation received by receiver elements T1, T2and T3 generates a photo-current which influences the voltage at theworking resistors. These voltages are amplified in the operationalamplifiers 20, 21 and 22. The outputs of the operational amplifiers 20,21 and 22 are connected via a diode network D₁ -D₆ to a centrallyprovided working resistor 40. The diodes are polarized andinterconnected so that the positive active voltages are brought to theupper point of the working resistor 40 and the negative active voltagesare brought to the base point of the working resistor. Thus, a maximumdifferential voltage is generated at working resistor 40 between themaximum highest positive voltage and the maximum lowest negativevoltage. The positive value is transmitted via an amplifier 38 to adifferential amplifier 41, while the negative value is brought to thesame differential amplifier 41 via an amplifier 39. The voltage at theoutput of the differential amplifier 41 corresponds to the proportionalpart of the yarn store on the storing surface. The voltage at the outputof the differential amplifier 41 is supplied via a diode and a resistornetwork to a comparator 43. The desired value of the yarn store size canbe adjusted at a potentiometer 44. Comparator 43 supplies the controldevice of drive motor 15 with the commands: Run or Stop.

The output signal of a sensor element S(D1, T1) is additionally takenacross 14 at the operational amplifier 20 and is supplied to a circuitpart D as well as to a parallel circuit part E.

A line 24 connects point 23 to one input of a comparator 26, the otherinput of which comparator 26 is connected to an adjustable thresholdvalue member 27. The output of comparator 26 is connected to a combiningor monitoring device V, which, preferably, is integrated into amicroprocessor M. Microprocessor M has connected thereto an alarm-signalemitter 4 and, optionally, a switch-off member 11. The parallel circuitpart E branches off at point 23 with a line 25 being connected to oneinput of a second comparator 28, the other input of which is connectedto a second threshold value member 29. The output of the secondcomparator 28 is also connected to device V. Threshold value member 27is set to a low threshold value, which corresponds to a signal levelbelow which the sensor device 7 would no longer be able to function,e.g. due to deteriorated light transmission quality. By contrast,threshold value member 29 is set to a higher threshold valuerepresenting a just barely acceptable deterioration of the lighttransmission quality, at which deterioration the sensor device is stillable to operate correctly, but removal of the contaminants influencingthe light transmission quality is already advisable.

In circuit part D, a rotational speed signal which represents the speedof drum 1 is generated on the basis of the output signal. The speedsignal is provided via the device V in the microprocessor M and isadapted to be used for evaluation. The microprocessor compares in anequivalency logic the presence of both signals from comparators 28 and26. In case both signals become unequal or one of the signals fails toappear, an alarm must be initiated.

A test signal is formed synchronously and essentially at the same timeand with the same signal level as the output signal. Since thresholdvalue member 29 is set to a higher threshold value than threshold valuemember 27, the test signal fails to appear at the device V as soon asits signal level falls below the threshold value. The signal emitter 4is activated by means of microprocessor M in order to, preferably, emitan optical or acoustical signal. If the contaminants are not removed,the microprocessor M may then activate the switch-off member 11 as soonas the speed signal also fails to appear, and may switch off the yarnfeeding device and the textile machine in order to avoid any emptying ofthe drum 1.

FIG. 4 illustrates a variant of the circuit part D and the parallelcircuit part E. A voltage divider, consisting of resistors 32, 33, 34,is provided in line 14. At point 35 between resistors 32 and 33, line 24branches off to one input of comparator 26. At point 37 between theresistors 33 and 34, line 25 branches off to one input of the secondcomparator 28. The signal level (voltage level) of the output signal islower at point 37 (test signal) than at point 35. Each second input ofthe first and second comparators 26, 28 is connected to a commonthreshold value member 36 set to a predetermined threshold value (areference voltage). This threshold value 36 is precisely adjusted to thepoint at which the contamination reaches a limit at which thecontamination is just barely acceptable, but too high for the signallevel of the test signal. By means of the voltage divider 32, 33, 34,comparator 28 switches at a higher threshold than comparator 26. In casethe sensor device 7 is contaminated, accordingly, the comparator 28 isno longer able to switch. By means of the equivalency examination of theoutput voltages of the comparators 26, 28 the microprocessor Mdetermines that an alarm signal has to be emitted, and the alarm signalemitter 4 is activated.

For a better understanding of the above-mentioned testing routine;reference is made to FIGS. 5, 5A, 5B and 5C. FIG. 5 illustrates in a Uvs. t diagram the object-output signal 38' in line 14, and how same isgenerated by the sensors S, D7, T1 depending upon the passing of thecircumferential sections 8, 9 or the surface areas A, B, which aredifferent from each other. At the two first signal levels the lighttransmission quality is still excellent. Starting with the third signallevel in FIG. 5, the quality of the light transmission decreases. Withincontrol circuit L according to FIG. 3, a signal 39' is present, as shownin the diagram of FIG. 5A. The threshold value set at the thresholdvalue member 27 is indicated by U1. At the output of comparator 26 asignal train C occurs according to FIG. 5C. By contrast, at the outputof comparator 28 a signal train G occurs according to FIG. 5C. Afterpoint X in time, signal train G is no longer present. An examination ofequality of the signal trains leads to a logical signal train H in FIG.5C. At point X in time microprocessor M activates the alarm signalmember 4.

Threshold value U2 in FIG. 5A represents a just barely acceptabledeterioration of the scanning conditions, i.e. the light transmissionquality, at which the sensor device 7 is able to still correctlyoperate, as it is shown in the lower part of FIG. 5A, by means of thestill existing signal 39' after point X in time and by signal train C inFIG. 5C. It should be noted that the light transmission quality normallydecreases during an essentially longer period of time than can bederived from FIGS. 5, 5A, 5B, 5C. These figures are schematic withrespect to the time duration and only serve to aid in understanding.

The diagram according to FIG. 5B corresponds to the variant according toFIG. 4. In the lower part of FIG. 5B, output signal 39" is identical tothe output signal 39' of FIG. 5A. The threshold value U1 equals thethreshold value U1 of FIG. 5A. In the upper part of FIG. 5B it can beseen that by influence of the voltage divider the signal levels of atest signal 40" derived from the object-output signal 38' are each lowerthan the signal level of the signal 39"; for test signal 40", however,the same threshold value U1 is considered as for signal 39". The firstthree signal levels of the test signal 40" are sufficiently high to passthe second comparator 28. The fourth signal level is, however, lowerthan the threshold value U1, so that then test signal 40" fails toappear at the combining device V and the alarm signal is generated.

By means of the circuit part D and the parallel circuit part E and thecomponents arranged therein, an checking device is created forevaluating the correspondence between the test signal and the speedsignal. This checking device can be realized very simply in themicroprocessor M by software adaptation. The examination of the qualityof the light transmission is only carried out when the drive motor isdriven for replenishing the yarn store, since with a stopped drum thesensor device only scans the yarn and does not see the reflecting bars Rand cannot reliably judge the quality of the reflecting lighttransmission.

The method can also be used for other physical scanning principles, e.g.when scanning by means of sound, induction, magnetism, capacitance, orthe like.

Although a particular preferred embodiment of the invention has beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modifications of the disclosed apparatus, includingthe rearrangement of parts, lie within the scope of the presentinvention.

I claim:
 1. A method of monitoring the scanning conditions duringcontrol of a yarn feeding device for feeding yarn to a textile machine,the yarn feeding device including a surface for storing yarn inwindings, a drive motor for winding yarn onto the storing surface toreplenish same with yarn, a sensor device including at least one sensororiented towards a scanning zone defined by the yarn feeding device, anda control circuit connected to the sensor device, said methodincluding:(1) generating an object-output signal with the sensor tocontrol the drive motor in response to a movement or the presence orabsence of an object in the scanning zone, the signal level of theobject-output signal being dependent upon the quality of the scanningconditions; (2) generating a test signal based upon the object-outputsignal substantially synchronously with said step (1); (3) comparing thesignal level of the test signal with a threshold value corresponding toa just barely acceptable deterioration level of the scanning conditions;and (4) generating an alarm signal when the signal level of the testsignal falls below the threshold value.
 2. The method according to claim1 including monitoring the test signal and the object-output signal, andwhen the test signal is no longer present and the object-output signalis present, generating the alarm signal.
 3. The method according toclaim 1 including detecting, with the sensor device, the absence of yarnin the scanning zone and thereupon rotating the storing surface with thedrive motor to wind yarn thereon, and generating a signal correspondingto a rotational speed of the storing surface, the test signal and thespeed signal both being generated from the object-output signal of thesensor device.
 4. The method according to claim 3 wherein the thresholdvalue is a first threshold value, said method including providing thespeed signal and the test signal with essentially equal signal levels,comparing the signal level of the test signal with the first thresholdvalue and comparing the signal level of the speed signal with a secondthreshold value lower than the first threshold value and correspondingto a worse deterioration level of the scanning conditions as compared tothe first threshold value, and generating the alarm signal when the testsignal is no longer present, and generating a signal for switching offat least one of the yarn feeding device and the textile machine whenboth the test signal and the speed signal are no longer present.
 5. Themethod according to claim 3 including providing the test signal with asignal level which is lower than the signal level of the speed signal,and comparing both the test signal level and the speed signal level withthe threshold value.
 6. The method according to claim 3 includinggenerating the alarm signal when the test signal is no longer presentand the speed signal is present.
 7. The method according to claim 1including providing an optoelectronic sensor device foroptoelectronically scanning objects in the scanning zone.
 8. A yarnfeeding device for feeding yarn to a textile machine comprising:ahousing; a surface for storing yarn; a controllable drive motor disposedfor driving a winding element which winds yarn onto the storing surfaceto define a yarn store of several windings; a stationarysignal-generating sensor device mounted on the housing and orientedtowards at least one scanning zone defined on the storing surface forsensing the motion or the presence or absence of an object in thescanning zone; and a control circuit which processes an object-outputsignal generated by the sensor device and controls the drive motor basedupon the object-output signal, the signal level of the object-outputsignal being dependent upon the quality of the scanning conditions atthe sensor device;said control circuit including a parallel circuit partfor generating and evaluating a test signal which is generatedsubstantially synchronously with and is formed from the object-outputsignal, said control circuit being connected to an alarm-signal emitterwhich is actuable upon a change in the signal level of the test signalwhich corresponds to a just barely acceptable deterioration level of thescanning conditions at the sensor device.
 9. The yarn feeding deviceaccording to claim 8, wherein the winding element is a drum whichdefines the storing surface thereon and is rotatably drivable by thedrive motor, said drum having surface areas adjacent the scanning zonewhich alternate with one another circumferentially along the drum andhave scanning properties which are different from one another, whereinwhen yarn is absent from the scanning zone and the drum rotates relativeto the sensor device, the surface areas are scanned by the sensor deviceas the object, said sensor device generating the object-output signalduring the scanning of the surface areas when the drum is rotating, saidobject-output signal corresponding to a rotational speed of the drum,the parallel circuit part including a threshold value member which setsa threshold value corresponding to a just barely acceptabledeterioration level of the scanning conditions at the sensor device andat least one of the surface areas being scanned, said parallel circuitpart being adapted to compare the test signal with the threshold valueset by the threshold value member.
 10. The yarn feeding device accordingto claim 8 wherein the sensor device comprises an optoelectronic sensordevice for generating the object-output signal having a signal leveldependent upon the light transmission quality of the sensor device. 11.The yarn feeding device according to claim 8 wherein the winding elementis a drum which defines the storing surface and is rotatably driven bythe drive motor, the control circuit includes a circuit part forderiving a signal from the object-output signal which corresponds to arotational speed of the drum, the circuit part and the parallel circuitpart being jointly connected to a microprocessor, and the microprocessorincluding a program routine which actuates an alarm signal when thespeed signal is present and the test signal is absent.
 12. The yarnfeeding device according to claim 11 wherein the microprocessor isconnected to a switch which is actuated by the program routine to switchoff the yarn feeding device when, in the activated state of the drivemotor, the speed signal is absent.
 13. The yarn feeding device accordingto claim 11 wherein the circuit part and the parallel circuit part arejointly connected to a voltage divider, the circuit part is connected toa first input of a first comparator, an output of said first comparatorbeing connected to the microprocessor, and a second input of said firstcomparator is connected to a first threshold value member which sets afirst threshold value comprising a first reference voltage, and theparallel circuit part is connected to a first input of a secondcomparator, an output of said second comparator being connected to themicroprocessor, and a second input of said second comparator isconnected to a second threshold value member which sets a secondthreshold value comprising a second reference voltage which is higherthan said first reference voltage.
 14. The yarn feeding device accordingto claim 13 wherein said control circuit is adapted to provide said testsignal and said speed signal with essentially equal signal levels. 15.The yarn feeding device according to claim 11 wherein said controlcircuit comprises a voltage divider including a resistor, the circuitpart is connected to said voltage divider upstream of said resistor andthe parallel circuit part is connected to said voltage dividerdownstream of said resistor, said circuit part is connected to an inputof a first comparator and an output of said first comparator isconnected to the microprocessor, the parallel circuit part is connectedto an input of a second comparator and an output of said secondcomparator is connected to the microprocessor, and a second input ofeach of the first and second comparators is connected to a commonthreshold value member which sets a single threshold value comprising asingle reference voltage.
 16. The yarn feeding device according to claim4 wherein the sensor device is provided with a plurality ofoptoelectronic sensors which are circumferentially spaced from oneanother in relation to the drum, each said sensor including atransmitter and a receiver element associated with said transmitter, andboth the circuit part and the parallel circuit part are connected toonly one of said optoelectronic sensors.